1. Field of the Invention
The present invention relates to processes for the preparation of halonitroalkanes, e.g. bromonitromethane, and particularly to an improved method for the recovery of halonitroalkanes from a reaction mixture.
2. Description of the Prior Art
Processes for the production of halonitroalkanes such as bromonitromethane are well known in the art. The conventional method for preparing bromonitromethane is disclosed in U.S. Pat. No. 2,632,776, issued to Slagh on Mar. 24, 1953. This process comprises first reacting nitromethane with an alkali metal hydroxide, calcium hydroxide or barium hydroxide to yield the nitromethane salt in solution. Bromine is then admixed with the salt solution and the bromonitromethane readily forms and is recovered by steam distillation. This process is reported to be useful in preparing bromonitromethane in yields of 70-90% of theoretical.
Similar processes for the preparation of halonitroalkanes are described in U.S. Pat. Nos. 4,922,030 and 5,043,489, issued to Nocito et al. on May 1, 1990 and Aug. 27, 1991, respectively. The disclosures of the Slagh and Nocito et al. patents relating to the basic two-step process for preparing halonitroalkanes are hereby incorporated.
The prior art has recognized the need to control various parameters of the basic reaction scheme in order to obtain halonitroalkanes in high yield. The Slagh patent recognizes the importance of controlling a number of factors, including the proportions of nitroalkane, alkali, water and bromine used; the temperatures at which the salt formation and bromination are performed; the time between formation of the salt and its subsequent bromination; and the rate of addition of bromine. The Nocito et al. patents also emphasize many of the foregoing considerations, and also suggest the use after the second step of a compound, such as sodium bisulfite, to destroy unreacted bromine to avoid over bromination of the nitroalkane.
One method of recovery of the prepared halonitroalkane is by extraction, for example using methylene chloride (MDC). The MDC layer is isolated and the MDC is removed under reduced pressure at temperatures below about 30.degree. C., and no product degradation occurs. However, this introduces a solvent into an otherwise organic solvent-free system, and adds additional costs to the process.
Alternatively, the halonitroalkane may be recovered by azeoptropic distillation, e.g. steam distillation. However, it has been determined that the recovery of halonitroalkanes by steam distillation of the reaction effluent results in loss of some of the product, resulting in reduced yield. There has therefore remained a need to provide for an improved recovery of the formed halonitroalkane in order to obtain increased overall yield from this conventional, two-step formation process.